In the forthcoming evolution of the mobile cellular standards like Global System for Mobile communications (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) will be used. Furthermore, in order to have a smooth migration from the existing cellular systems to a new high capacity and high data rate system in existing radio spectrum, the new system will probably have to be able to operate in a flexible bandwidth. A proposal for such a new flexible cellular system is Third Generation Partnership Project Long Term Evolution (3GPP LTE) that can be seen as an evolution of the WCDMA standard. This system will use OFDM as multiple access technique, thus called OFDMA, in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported for the largest bandwidth, and such data rates will be possible to reach using Multiple-Input-Multiple Output (MIMO) scheme in the down-link. In order to optimize the throughput it is preferable that the terminal estimates the signal-to-interference ratio (SIR) properly, used for feedback of a channel quality index (CQI). Furthermore, it is also preferable to determine whether single or multiple streams, i.e. MIMO, should be transmitted.
Typically SIR used for CQI is estimated by estimating a pilot signal power divided to the total interference and thermal noise power,SIR=C/(I+N)
where C is pilot signal power, I is interference power, and N is noise power, and then correct the estimate using some possible bias compensation. However, since 3GPP LTE is a packet-based system, the interference level could vary significantly from sub-frame to sub-frame. However, in thermal noise limited scenarios, the interference is almost constant from sub-frame to sub-frame, hence making the CQI estimates more reliable, in terms of that the CQI will be valid for many sub-frames. Standard SIR estimates do not count for the differences between interference and noise making it hard to discriminate between interference and noise.
US 2006/0133549 A1 discloses a method and system for receiving a plurality of transmission signals. The received transmission signals are detected to determine whether they are dominated by noise or interference. If they are dominated by noise, a first set of weights is selected for receiver weighting, and consequently, if they are dominated by interference, a second set of weights is selected. Detection is performed by determining level of correlation between the received transmission signals.
Furthermore, in pre-coding vector computations, used for feedback of pre-coding information to the base station in MIMO schemes, prior art solutions typically computes the pre-coding vectors for single and multiple streams based on prior art SIR estimate which again does not count for whether the noise is interference or thermal noise.
Therefore, there is a need for an estimate of a carrier-to-interference ratio (C/I).